C++ LANGUAGE FRIEND FUNCTION

COMPUTER LANGUAGES HTML,C,C++.JAVA,.NET AND MULTIMEDIA basics and programs click home button

C++ LANGUAGE FRIEND FUNCTION

Friend Functions 


A friend function of a class is defined outside that class' 
scope but it has the right to access all private and protected 
members of the class. Even though the prototypes for friend 
functions appear in the class definition, friends are not member functions. 
A friend can be a function, function template, or member function, 
or a class or class template, in which case the entire class and all 
of its members are friends. 
To declare a function as a friend of a class, precede the function 
prototype in the class definition with keyword friend as follows: 

class Box 
{ 
   double width; 
public: 
   double length; 
   friend void printWidth( Box box ); 
   void setWidth( double wid ); 
}; 
To declare all member functions of class ClassTwo as friends 
of class ClassOne, place a following declaration in the definition 
of class ClassOne: 

friend class ClassTwo; 
Consider the following program: 
#include <iostream> 
  
using namespace std; 
  
class Box 
{ 
   double width; 
public: 
   friend void printWidth( Box box ); 
   void setWidth( double wid ); 
}; 
 
// Member function definition 
void Box::setWidth( double wid ) 
{ 
    width = wid; 
} 
 
// Note: printWidth() is not a member function of any class. 
void printWidth( Box box ) 
{ 
   /* Because printWidth() is a friend of Box, it can 
    directly access any member of this class */ 
   cout << "Width of box : " << box.width <<endl; 
} 
  
// Main function for the program 
int main( ) 
{ 
   Box box; 
  
   // set box width without member function 
   box.setWidth(10.0); 
    
   // Use friend function to print the wdith. 
   printWidth( box ); 
  
   return 0; 
} 
When the above code is compiled and executed, it produces the following result: 
Width of box : 10